I do wonder about the motivations of S&P. However, if their goal is to hurt Obama, I'm not sure that they've got the timing right. It seems likely that the Tea Party's brinksmanship could take a lot of the blame for this. Americans are warming up to revenue increases, particularly on the wealthy. I almost wonder if Obama didn't let the Bush tax cut extension on the $250k earners slide to 2012 so that he could campaign on it.
That said, playing chicken with the debt-ceiling is probably as good a reason for a down-grade if any. Not that S&P has a good track record.
At any rate, I plan to buy some stocks today. Emotional drops in the market are a gift.